The HIV envelope protein, gp120 mediates entry of the virus into CD4+ human cells. gp120 is also the sole viral protein against which neutralizing antibodies are elicited. For these reasons gp120 is an important protein both in the development of therapeutic agents designed to prevent infection of human cells, and as a principal component of an AIDS vaccine. One of the key events in the viral replication cycle is the binding of gp120, expressed on the surface of virions, to CD4 receptors expressed on human T-cells. When gp120 binds to the CD4 receptor its structure (conformation) changes in a dramatic fasion. The principal purpose of this project is to design and characterize a recombinant protein derivative of the CD4 receptor that effectively locks gp120 into the CD4-bound conformation. The development and characterization of such a reagent will provide fundamental information relevant to the design of effective entry inhibitors and vaccines. Monomeric soluble CD4 (sCD4) has been evaluated as a decoy drug to absorb sufficient amounts of HIV in vivo to limit the progression of disease. Unfortunately, sCD4 was ineffective in human trials. We hypothesized that a sCD4-based molecule that was large, that bound multiple gp120s simultaneously, and that was highly avid to gp120 could overcome the defects of monomeric sCD4 and effectively inhibit HIV entry into target cells. Therefore, we constructed a polymeric CD4-IgG1 fusion protein with a very large hydrodynamic radius of 12 nm and the capacity to bind at least 10 gp120 subunits with binding kinetics that suggested a highly avid interaction to virion-associated envelope; the protein was called D1D2-Igatp. Unlike sCD4, this protein did not enhance viral replication at suboptimal concentrations. In viral neutralization assays, D1D2-Igatp inhibited HIV replication at levels comparable to the most potent neutralizing antibodies available. In addition, it has been determined that D1D2-Igatp crosslinks CD16 receptors on NK cells and activates those cells. The dual specificity of this molecule for both HIV envelope and CD16 may promote NK cell-mediated killing of HIV-infected cells. These observations may aid in the design of new therapeutic strategies for HIV infection. With respect to vaccine development, D1D2-Igatp binds to gp120 in a higly avid manner, and in doing so effectively locks gp120 in an alternate confomation. This alternate conformation exposes epitopes on gp120 that may elicit more broadly cross-reactive neutralizing antibody responses. We have tested complexes of D1D2-Igatp bound to recombinant gp120, and have shown in animals that the immune response to this complex is considerably more broadly cross-reactive than that to the ualtered gp120.